It is often necessary and/or desirable to accurately itermine the state-of-charge of a power supply such as a battery or battery pack. Heretofore, the most common methods for indicating state-of-charge have been through the use of ampere-hour meters, specific gravity measurements, or voltage analyzers.
Of these, ampere-hour meters simply integrate the discharge current and cannot compensate for changes in battery capacity as a function of current discharge history. Specific gravity measurements, on the other hand, are normally made with a hydrometer and must be applied to each cell to achieve accurate measurement. Since specific gravity also lags the available capacity, results gained through use of a hydrometer can be somewhat inaccurate and since the expense of monitoring each cell is large, determination of state-of-charge by specific gravity is seldom used for continuous indications (as opposed to use at the start of a cycle to determine the initial state-of-charge). Voltage analyzers measure the terminal voltage and determine state-of-charge in various manners. With respect to such analyzers, polarization effects (caused by charging or fast discharging) can adversely change the indication of the analyzer meters, and most such analyzers tend to be only uni-directional (work accurately on discharge only) with no provision for aging effects.
A number of patents have been issued which describe various devices and/or methods for predicting the state-of-charge of batteries (particularly lead-acid batteries) for vehicle applications. Among such patents are U.S. Pat. Nos. 3,778,702; 3,895,284; 3,971,980; 3,997,888; 4,012,681; 4,017,724; 4,024,523; 4,025,916; 4,051,424; 4,052,717; 4,080,560; 4,114,083; and 4,153,867.
Monitoring and/or indicating devices heretofore known and/or utilized, while describing various state-of-charge meters and/or methods, have not proved to be completely satisfactory and have limitations which can be eliminated, or at least minimized, by new realizations and concepts in this field.